Physics:Atomic units

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Short description: System of measurement

The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested and named by the physicist Douglas Hartree.[1] Atomic units are often abbreviated "a.u." or "au", not to be confused with similar abbreviations used for astronomical units, arbitrary units, and absorbance units in other contexts.

Motivation

In the context of atomic physics, using the atomic units system can be a convenient shortcut, eliminating unnecessary symbols and numbers with very small orders of magnitude. For example, the Hamiltonian operator in the Schrödinger equation for the helium atom with standard quantities, such as when using SI units, is[2]:{{{1}}}

[math]\displaystyle{ \hat{H} = - \frac{\hbar^2}{2m_\text{e}} \nabla_1^2 - \frac{\hbar^2}{2m_\text{e}} \nabla_2^2 - \frac{2e^2}{4\pi\epsilon_0 r_1} - \frac{2e^2}{4\pi\epsilon_0 r_2} + \frac{e^2}{4\pi\epsilon_0 r_{12}} , }[/math]

but adopting the convention associated with atomic units that transforms quantities into dimensionless equivalents, it becomes

[math]\displaystyle{ \hat{H} = - \frac{1}{2} \nabla_1^2 - \frac{1}{2} \nabla_2^2 - \frac{2}{r_1} - \frac{2}{r_2} + \frac{1}{r_{12}} . }[/math]

In this convention, the constants [math]\displaystyle{ \hbar }[/math], [math]\displaystyle{ m_\text{e} }[/math], [math]\displaystyle{ 4 \pi \epsilon_0 }[/math], and [math]\displaystyle{ e }[/math] all correspond to the value [math]\displaystyle{ 1 }[/math] (see § Definition below). The distances relevant to the physics expressed in SI units are naturally on the order of [math]\displaystyle{ 10^{-10}\,\mathrm{m} }[/math], while expressed in atomic units distances are on the order of [math]\displaystyle{ 1 a_0 }[/math] (one Bohr radius, the atomic unit of length). An additional benefit of expressing quantities using atomic units is that their values calculated and reported in atomic units do not change when values of fundamental constants are revised. The fundamental constants are built into the conversion factors between atomic units and SI.

History

Hartree defined units based on three physical constants:[1]:91

Both in order to eliminate various universal constants from the equations and also to avoid high powers of 10 in numerical work, it is convenient to express quantities in terms of units, which may be called 'atomic units', defined as follows:
Unit of length, [math]\displaystyle{ a_\text{H} = h^2 \,/\, 4 \pi^2 m e^2 }[/math], on the orbital mechanics the radius of the 1-quantum circular orbit of the H-atom with fixed nucleus.
Unit of charge, [math]\displaystyle{ e }[/math], the magnitude of the charge on the electron.
Unit of mass, [math]\displaystyle{ m }[/math], the mass of the electron.

Consistent with these are:

Unit of action, [math]\displaystyle{ h \,/\, 2 \pi }[/math].
Unit of energy, [math]\displaystyle{ e^2 / a_\text{H} = 2 h c R = }[/math] [...]
Unit of time, [math]\displaystyle{ 1 \,/\, 4 \pi c R }[/math].
 

Here, the modern equivalent of [math]\displaystyle{ R }[/math] is the Rydberg constant [math]\displaystyle{ R_\infty }[/math], of [math]\displaystyle{ m }[/math] is the electron mass [math]\displaystyle{ m_\text{e} }[/math], of [math]\displaystyle{ a_\text{H} }[/math] is the Bohr radius [math]\displaystyle{ a_0 }[/math], and of [math]\displaystyle{ h / 2 \pi }[/math] is the reduced Planck constant [math]\displaystyle{ \hbar }[/math]. Hartree's expressions that contain [math]\displaystyle{ e }[/math] differ from the modern form due to a change in the definition of [math]\displaystyle{ e }[/math], as explained below.

In 1957, Bethe and Salpeter's book Quantum mechanics of one-and two-electron atoms[3] built on Hartree's units, which they called atomic units abbreviated "a.u.". They chose to use [math]\displaystyle{ \hbar }[/math], their unit of action and angular momentum in place of Hartree's length as the base units. They noted that the unit of length in this system is the radius of the first Bohr orbit and their velocity is the electron velocity in Bohr's model of the first orbit.

In 1959, Shull and Hall[4] advocated atomic units based on Hartree's model but again chose to use [math]\displaystyle{ \hbar }[/math] as the defining unit. They explicitly named the distance unit a "Bohr radius"; in addition, they wrote the unit of energy as [math]\displaystyle{ H = m e^4 / \hbar^2 }[/math] and called it a Hartree. These terms came to be used widely in quantum chemistry.[5]:349

In 1973 McWeeny extended the system of Shull and Hall by adding permittivity in the form of [math]\displaystyle{ \kappa_0 = 4 \pi \epsilon_0 }[/math] as a defining or base unit.[6][7] Simultaneously he adopted the SI definition of [math]\displaystyle{ e }[/math] so that his expression for energy in atomic units is [math]\displaystyle{ e^2 / (4 \pi \epsilon_0 a_0) }[/math], matching the expression in the 8th SI brochure.[8]

Definition

A set of base units in the atomic system as in one proposal are the electron rest mass, the magnitude of the electronic charge, the Planck constant, and the permittivity.[6] In the atomic units system, each of these takes the value 1; the corresponding values in the International System of Units[9]:132 are given in the table.

Table notes
dagger This choice of base units, which is essentially arbitrary, is McWeeny's proposal.
Template:Double dagger W represents the dimensions of energy, ML2T−2.[6]
⹋ In the 'atomic units' column, the convention that uses dimensionless equivalents has been applied.
Base atomic unitsdagger
Symbol and Name Quantity (dimensions)Template:Double dagger Atomic units⹋ SI units
[math]\displaystyle{ \hbar }[/math], reduced Planck constant action (ML2T−1) 1 1.054571817...×10−34 J⋅s [10]
[math]\displaystyle{ e }[/math], elementary charge charge (Q) 1 1.602176634×10−19 C [11]
[math]\displaystyle{ m_\text{e} }[/math], electron rest mass mass (M) 1 9.1093837015(28)×10−31 kg [12]
[math]\displaystyle{ 4 \pi \epsilon_0 }[/math], permittivity permittivity (Q2W−1L−1) 1 1.11265005545(17)×10−10 F⋅m−1 [13]

Units

Three of the defining constants (reduced Planck constant, elementary charge, and electron rest mass) are atomic units themselves – of action,[14] electric charge,[15] and mass,[16] respectively. Two named units are those of length (Bohr radius [math]\displaystyle{ a_0 \equiv 4 \pi \epsilon_0 \hbar^2 / m_\text{e} e^2 }[/math]) and energy (hartree [math]\displaystyle{ E_\text{h} \equiv \hbar^2 / m_\text{e} a_0^2 }[/math]).

Defined atomic units
Atomic unit of Expression Value in SI units Other equivalents
1st hyperpolarizability [math]\displaystyle{ e^3 a_0^3 / E_\text{h}^2 }[/math] 3.2063613061(15)×10−53 C3⋅m3⋅J−2 [17]
2nd hyperpolarizability [math]\displaystyle{ e^4 a_0^4 / E_\text{h}^3 }[/math] 6.2353799905(38)×10−65 C4⋅m4⋅J−3 [18]
action [math]\displaystyle{ \hbar }[/math] 1.054571817...×10−34 J⋅s [19]
charge [math]\displaystyle{ e }[/math] 1.602176634×10−19 C [20]
charge density [math]\displaystyle{ e/a_0^3 }[/math] 1.08120238457(49)×1012 C⋅m−3 [21]
current [math]\displaystyle{ e E_\text{h} / \hbar }[/math] 6.623618237510(13)×10−3 A [22]
electric dipole moment [math]\displaystyle{ e a_0 }[/math] 8.4783536255(13)×10−30 C⋅m [23] ≘ 2.541746473 D
electric field [math]\displaystyle{ E_\text{h} / e a_0 }[/math] 5.14220674763(78)×1011 V⋅m−1 [24] 5.14220674763(78) GV⋅cm−1, 51.4220674763(78) V⋅Å−1
electric field gradient [math]\displaystyle{ E_\text{h} / e a_0^2 }[/math] 9.7173624292(29)×1021 V⋅m−2 [25]
electric polarizability [math]\displaystyle{ e^2 a_0^2 / E_\text{h} }[/math] 1.64877727436(50)×10−41 C2⋅m2⋅J−1 [26]
electric potential [math]\displaystyle{ E_\text{h} / e }[/math] 27.211386245988(53) V [27]
electric quadrupole moment [math]\displaystyle{ e a_0^2 }[/math] 4.4865515246(14)×10−40 C⋅m2 [28]
energy [math]\displaystyle{ E_\text{h} }[/math] 4.3597447222071(85)×10−18 J [29] [math]\displaystyle{ 2 h c R_\infty }[/math], [math]\displaystyle{ \alpha^2 m_\text{e} c^2 }[/math], 27.211386245988(53) eV [30]
force [math]\displaystyle{ E_\text{h} / a_0 }[/math] 8.2387234983(12)×10−8 N [31] 82.387 nN, 51.421 eV·Å−1
length [math]\displaystyle{ a_0 }[/math] 5.29177210903(80)×10−11 m [32] [math]\displaystyle{ \hbar / \alpha m_\text{e} c }[/math], 0.529177210903(80) Å
magnetic dipole moment [math]\displaystyle{ \hbar e / m_\text{e} }[/math] 1.85480201566(56)×10−23 J⋅T−1 [33] [math]\displaystyle{ 2 \mu_\text{B} }[/math]
magnetic flux density [math]\displaystyle{ \hbar/e a_0^2 }[/math] 2.35051756758(71)×105 T [34] ≘ 2.35051756758(71)×109 G
magnetizability [math]\displaystyle{ e^2 a_0^2 / m_\text{e} }[/math] 7.8910366008(48)×10−29 J⋅T−2 [35]
mass [math]\displaystyle{ m_\text{e} }[/math] 9.1093837015(28)×10−31 kg [36]
momentum [math]\displaystyle{ \hbar/a_0 }[/math] 1.99285191410(30)×10−24 kg·m·s−1 [37]
permittivity [math]\displaystyle{ e^2 / a_0 E_\text{h} }[/math] 1.11265005545(17)×10−10 F⋅m−1 [38] [math]\displaystyle{ 4 \pi \epsilon_0 }[/math]
time [math]\displaystyle{ \hbar / E_\text{h} }[/math] 2.4188843265857(47)×10−17 s [39]
velocity [math]\displaystyle{ a_0 E_\text{h} / \hbar }[/math] 2.18769126364(33)×106 m⋅s−1 [40] [math]\displaystyle{ \alpha c }[/math]

[math]\displaystyle{ c }[/math]: speed of light, [math]\displaystyle{ \epsilon_0 }[/math]: vacuum permittivity, [math]\displaystyle{ R_\infty }[/math]: Rydberg constant, [math]\displaystyle{ h }[/math]: Planck constant, [math]\displaystyle{ \alpha }[/math]: fine-structure constant, [math]\displaystyle{ \mu_\text{B} }[/math]: Bohr magneton, ≘: correspondence

Conventions

Different conventions are adopted in the use of atomic units, which vary in presentation, formality and convenience.

Explicit units

  • Many texts (e.g. Jerrard & McNiell,[7] Shull & Hall[4]) define the atomic units as quantities, without a transformation of the equations in use. As such, they do not suggest treating either quantities as dimensionless or changing the form of any equations. This is consistent with expressing quantities in terms of dimensional quantities, where the atomic unit is included explicitly as a symbol (e.g. [math]\displaystyle{ m = 3.4~m_\text{e} }[/math], [math]\displaystyle{ m = 3.4~\text{a.u. of mass} }[/math], or more ambiguously, [math]\displaystyle{ m = 3.4~\text{a.u.} }[/math]), and keeping equations unaltered with explicit constants.[41]
  • Provision for choosing more convenient closely related quantities that are more suited to the problem as units than universal fixed units are is also suggested, for example based on the reduced mass of an electron, albeit with careful definition thereof where used (for example, a unit [math]\displaystyle{ H_M = \mu e^4 / \hbar^2 }[/math], where [math]\displaystyle{ \mu = m_\text{e} M / (m_\text{e} + M) }[/math] for a specified mass [math]\displaystyle{ M }[/math]).[4]

A convention that eliminates units

In atomic physics, it is common to simplify mathematical expressions by a transformation of all quantities:

  • Hartree suggested that expression in terms of atomic units allows us "to eliminate various universal constants from the equations", which amounts to informally suggesting a transformation of quantities and equations such that all quantities are replaced by corresponding dimensionless quantities.[1]:91 He does not elaborate beyond examples.
  • McWeeny suggests that "... their adoption permits all the fundamental equations to be written in a dimensionless form in which constants such as [math]\displaystyle{ e }[/math], [math]\displaystyle{ m }[/math] and [math]\displaystyle{ h }[/math] are absent and need not be considered at all during mathematical derivations or the processes of numerical solution; the units in which any calculated quantity must appear are implicit in its physical dimensions and may be supplied at the end." He also states that "An alternative convention is to interpret the symbols as the numerical measures of the quantities they represent, referred to some specified system of units: in this case the equations contain only pure numbers or dimensionless variables; ... the appropriate units are supplied at the end of a calculation, by reference to the physical dimensions of the quantity calculated. [This] convention has much to recommend it and is tacitly accepted in atomic and molecular physics whenever atomic units are introduced, for example for convenience in computation."
  • An informal approach is often taken, in which "equations are expressed in terms of atomic units simply by setting [math]\displaystyle{ \hbar = m_\text{e} = e = 4 \pi \epsilon_0 = 1 }[/math]".[41][42][43] This is a form of shorthand for the more formal process of transformation between quantities that is suggested by others, such as McWeeny.

Physical constants

Dimensionless physical constants retain their values in any system of units. Of note is the fine-structure constant [math]\displaystyle{ \alpha = {e^2} / {(4 \pi \epsilon_0)\hbar c} \approx 1/137 }[/math], which appears in expressions as a consequence of the choice of units. For example, the numeric value of the speed of light, expressed in atomic units, is [math]\displaystyle{ c = 1/\alpha\,\text{a.u.} \approx 137\,\text{a.u.} }[/math][44]:597

Some physical constants expressed in atomic units
Name Symbol/Definition Value in atomic units
speed of light [math]\displaystyle{ c }[/math] [math]\displaystyle{ (1/\alpha) \,a_0 E_\text{h}/\hbar \approx 137 \,a_0 E_\text{h}/\hbar }[/math]
classical electron radius [math]\displaystyle{ r_\text{e}=\frac{1}{4\pi\epsilon_0}\frac{e^2}{m_\text{e} c^2} }[/math] [math]\displaystyle{ \alpha^2 \,a_0 \approx 0.0000532 \,a_0 }[/math]
reduced Compton wavelength
of the electron		 ƛe [math]\displaystyle{ = \frac{\hbar}{m_\text{e} c} }[/math] [math]\displaystyle{ \alpha \,a_0 \approx 0.007297 \,a_0 }[/math]
proton mass [math]\displaystyle{ m_\text{p} }[/math] [math]\displaystyle{ \approx 1836 \,m_\text{e} }[/math]

Bohr model in atomic units

Atomic units are chosen to reflect the properties of electrons in atoms, which is particularly clear in the classical Bohr model of the hydrogen atom for the bound electron in its ground state:

  • Mass = 1 a.u. of mass
  • Charge = −1 a.u. of charge
  • Orbital radius = 1 a.u. of length
  • Orbital velocity = 1 a.u. of velocity[44]:597
  • Orbital period = 2π a.u. of time
  • Orbital angular velocity = 1 radian per a.u. of time
  • Orbital momentum = 1 a.u. of momentum
  • Ionization energy = 1/2 a.u. of energy
  • Electric field (due to nucleus) = 1 a.u. of electric field
  • Lorentz force (due to nucleus) = 1 a.u. of force

References

[46]

  1. ↑ 1.0 1.1 1.2 Hartree, D. R. (1928), "The Wave Mechanics of an Atom with a Non-Coulomb Central Field. Part I. Theory and Methods", Mathematical Proceedings of the Cambridge Philosophical Society (Cambridge University Press) 24 (1): 89–110, doi:10.1017/S0305004100011919, Bibcode: 1928PCPS...24...89H, http://journals.cambridge.org/action/displayAbstract?aid=1733252 
  2. ↑ McQuarrie, Donald A. (2008). Quantum Chemistry (2nd ed.). New York, NY: University Science Books. 
  3. ↑ Bethe, Hans A.; Salpeter, Edwin E. (1957) (in en). Introduction. Units. Berlin, Heidelberg: Springer Berlin Heidelberg. pp. 2–4. doi:10.1007/978-3-662-12869-5_1. ISBN 978-3-662-12871-8. http://link.springer.com/10.1007/978-3-662-12869-5_1. 
  4. ↑ 4.0 4.1 4.2 Shull, H.; Hall, G. G. (1959). "Atomic Units". Nature 184 (4698): 1559. doi:10.1038/1841559a0. Bibcode: 1959Natur.184.1559S. 
  5. ↑ Levine, Ira N. (1991). Quantum chemistry. Pearson advanced chemistry series (4 ed.). Englewood Cliffs, NJ: Prentice-Hall International. ISBN 978-0-205-12770-2. 
  6. ↑ 6.0 6.1 6.2 McWeeny, R. (May 1973). "Natural Units in Atomic and Molecular Physics" (in en). Nature 243 (5404): 196–198. doi:10.1038/243196a0. ISSN 0028-0836. Bibcode: 1973Natur.243..196M. https://www.nature.com/articles/243196a0. 
  7. ↑ 7.0 7.1 Jerrard, H. G.; McNeill, D. B. (1992) (in en). Systems of units. Dordrecht: Springer Netherlands. pp. 3–8. doi:10.1007/978-94-011-2294-8_2. ISBN 978-0-412-46720-2. http://link.springer.com/10.1007/978-94-011-2294-8_2. 
  8. ↑ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 125, ISBN 92-822-2213-6, http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf . Note that this information is omitted in the 9th edition.
  9. ↑ "9th edition of the SI Brochure". BIPM. 2019. https://www.bipm.org/en/publications/si-brochure/. 
  10. ↑ "reduced Planck constant". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?hbar. 
  11. ↑ "elementary charge". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?e. 
  12. ↑ "electron mass". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?me. 
  13. ↑ "atomic unit of permittivity". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auperm. 
  14. ↑ "atomic unit of action". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ahbar. 
  15. ↑ "atomic unit of charge". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ae. 
  16. ↑ "atomic unit of mass". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ame. 
  17. ↑ "atomic unit of 1st hyperpolarizability". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auhypol. 
  18. ↑ "atomic unit of 2nd hyperpolarizability". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?au2hypol. 
  19. ↑ "atomic unit of action". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ahbar. 
  20. ↑ "atomic unit of charge". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ae. 
  21. ↑ "atomic unit of charge density". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aucd. 
  22. ↑ "atomic unit of current". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aucur. 
  23. ↑ "atomic unit of electric dipole moment". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auedm. 
  24. ↑ "atomic unit of electric field". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auefld. 
  25. ↑ "atomic unit of electric field gradient". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auefg. 
  26. ↑ "atomic unit of electric polarizability". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auepol. 
  27. ↑ "atomic unit of electric potential". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auep. 
  28. ↑ "atomic unit of electric quadrupole moment". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aueqm. 
  29. ↑ "atomic unit of energy". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ahr. 
  30. ↑ "Hartree energy in eV". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?hrev. 
  31. ↑ "atomic unit of force". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auforce. 
  32. ↑ "atomic unit of length". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Abohrrada0. 
  33. ↑ "atomic unit of magnetic dipole moment". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aumdm. 
  34. ↑ "atomic unit of magnetic flux density". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aumfd. 
  35. ↑ "atomic unit of magnetizability". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aumag. 
  36. ↑ "atomic unit of mass". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?Ame. 
  37. ↑ "atomic unit of momentum". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aumom. 
  38. ↑ "atomic unit of permittivity". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auperm. 
  39. ↑ "atomic unit of time". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?aut. 
  40. ↑ "atomic unit of velocity". CODATA 2018. http://physics.nist.gov/cgi-bin/cuu/Value?auvel. 
  41. ↑ 41.0 41.1 Pilar, Frank L. (2001). Elementary Quantum Chemistry. Dover Publications. p. 155. ISBN 978-0-486-41464-5. https://books.google.com/books?id=XpGM7r69LdkC&pg=PA155. 
  42. ↑ Bishop, David M. (1993). Group Theory and Chemistry. Dover Publications. p. 217. ISBN 978-0-486-67355-4. https://books.google.com/books?id=l4zv4dukBT0C&pg=PA217. 
  43. ↑ Drake, Gordon W. F. (2006). Springer Handbook of Atomic, Molecular, and Optical Physics (2nd ed.). Springer. p. 5. ISBN 978-0-387-20802-2. https://books.google.com/books?id=Jj-ad_2aNOAC&pg=PA5. 
  44. ↑ 44.0 44.1 Karplus, Martin; Porter, Richard Needham (1970), Atoms and Molecules: An Introduction for Students of Physical Chemistry, Netherlands: W. A. Benjamin 
  45. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named CODATA_2018
  46. ↑ "CODATA Internationally recommended 2018 values of the Fundamental Physical Constants". NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-08-31 (all values). http://physics.nist.gov/cuu/Constants/index.html. 



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